Opacity effect on extreme ultraviolet radiation from laser-produced tin plasmas

Opacity effects on extreme ultraviolet (EUV) emission from laser-produced tin (Sn) plasma have been experimentally investigated. An absorption spectrum of a uniform Sn plasma generated by thermal x rays has been measured in the EUV range (9-19 nm wavelength) for the first time. Experimental results indicate that control of the optical depth of the laser-produced Sn plasma is essential for obtaining high conversion to 13.5 nm-wavelength EUV radiation; 1.8% of the conversion efficiency was attained with the use of 2.2 ns laser pulses.     

Effect of focal spot size on in-band 13.5 nm extreme ultraviolet emission from laser-produced Sn plasma

The effect of focal spot size on in-band 13.5 nm extreme ultraviolet (EUV) emission from laser-produced Sn plasmas was investigated for an EUV lithography light source. Almost constant in-band conversion efficiency from laser to 13.5 nm EUV light was noted with focal spot sizes from 60 to 500 microm. This effect may be explained by the opacity of Sn plasmas. Optical interferometry showed that the EUV emission must pass through a longer plasma with higher density when the focal spot is large, and strong reabsorption of EUV light was confirmed by a dip located at 13.5 nm in the spectrum.     

激光等离子体13.5 nm极紫外光刻光源进展

半导体产业是高科技、信息化时代的支柱。光刻技术,作为半导体产业的核心技术之一,已成为世界各国科研人员的重点研究对象。本文综述了激光等离子体13.5 nm极紫外光刻的原理和国内外研究发展概况,重点介绍了其激光源、辐射靶材和多层膜反射镜等关键系统组成部分。同时,指出了在提高激光等离子体13.5 nm极紫外光源输出功率的研究进程中所存在的主要问题,包括提高转换效率和减少光源碎屑。特别分析了目前已实现百瓦级输出的日本Gigaphoton公司和荷兰的ASML公司的极紫外光源装置。最后对该项技术的发展前景进行了总结与展望。     

Intensity measurements of extreme ultraviolet radiation from helium plasma by photoelectron spectroscopy

Extreme ultraviolet radiation emitted from He plasma has been studied by means of photoelectron energy analyzer. Relative and absolute intensities of Lyman series of ionized helium have been measured. From these spectral intensities, the electron temperature of the plasma has been estimated at several tens of eV.     

Optimization of EUV radiation yield from laser-produced plasma

We optimize the conversion of laser energy into extreme ultraviolet (EUV) radiation by tailoring the laser parameters for a laser-produced plasma generated from 20 μm diameter water droplets. It is shown that mass-limited targets require careful adaption of laser-pulse energy and laser-pulse duration separately, rather than laser intensity, which seems to be adequate for bulk targets. The optimal pulse duration scales with the droplet radius, and the optimal pulse energy with the droplet volume. With optimized parameters, we obtain a conversion efficiency of 0.23% in 4π and 2.5% bandwidth for 13 nm radiation, the future EUV lithography light. Received: 16 July 2001 / Revised version: 25 September 2001 / Published online: 7 November 2001     

Analysis of extreme ultraviolet spectral profiles of laser-produced Cr plasmas

Radiation from laser-produced plasmas was examined as a potential wavelength calibration source for spectrographs in the extreme ultraviolet(EUV) region.Specifically, the EUV emission of chromium(Cr) plasmas was acquired via spatiotemporally resolved emission spectroscopy.With the aid of Cowan and flexible atomic code(FAC) structure calculations,and a comparative analysis with the simulated spectra, emission peaks in the 6.5–15.0 nm range were identified as 3 p–4 d, 5 d and 3 p–4 s transition lines from Cr~(5+)–Cr~(10+)ions.A normalized Boltzmann distribution among the excited states and a steady-state collisional-radiative model were assumed for the spectral simulations, and used to estimate the electron temperature and density in the plasma.The results indicate that several relatively isolated emission lines of highly charged ions would be useful for EUV wavelength calibration.     

An extreme ultraviolet radiation source based on plasma heated by millimeter range radiation

An extreme ultraviolet (EUV) radiation source based on a low-pressure discharge sustained in a magnetic trap by high-power electromagnetic radiation at a frequency of 75 GHz is discussed. The experimental radiation power in the wavelength range of 13.5 nm ±1% was as high as 50 W. The conversion of microwave radiation into EUV radiation was about 1%.     

Auto-cleaning of deposited tin debris in a laser-produced plasma extreme ultraviolet source by using a liquid jet target containing tin chloride

We have demonstrated auto-cleaning of deposited tin debris produced in a laser-plasma extreme ultraviolet (EUV) source by using a liquid jet target containing tin chloride solution. The use of double laser pulse irradiation improved the EUV conversion efficiency of 1.1%, which was 3.3 times as large as that obtained with single laser pulse irradiation. At an appropriate concentration of tin chloride, the amount of deposited debris was balanced out with that of sputtered and/or etched debris, resulting in a thickness of deposited debris less than 1 nm after 40?000 laser pulse irradiations.     

Ablation of polymers by focused EUV radiation from a table-top laser-produced plasma source

We have investigated ablation of polymers with radiation of 13.5 nm wavelength, using a table-top laser produced plasma source based on solid gold as target material. A Schwarzschild objective with Mo/Si multilayer coatings was adapted to the source, generating an EUV spot of 5 μm diameter with a maximum energy density of ∼1.3 J/cm². In combination with a Zirconium transmission filter, radiation of high spectral purity (2% bandwidth) can be provided on the irradiated spot. Ablation experiments were performed on PMMA, PTFE and PC. Ablation rates were determined for varying fluences using atomic force microscopy and white light interferometry. The slopes of these curves are discussed with respect to the chemical structure of the polymers. Additionally, the ablation behavior in terms of effective penetration depths, threshold fluences and incubation effects is compared to literature data for higher UV wavelength.     

Suppression of fast-ions from a laser-produced plasma

Fast-ion emission occurs when a target is irradiated with single 25 psec pulses from a Nd: YAG/glass laser but is suppressed when the same target is irradiated with a mode-locked pulse train at the same intensity. Possible explanations for this effect are given.     

Generation of strong coherent extreme ultraviolet radiation from the laser plasma produced on the surface of solid targets

We demonstrate the generation of high harmonics (up to the 65th order, λ=12.24 nm) of a Ti:sapphire laser radiation after the propagation of femtosecond laser pulses through the low-excited plasma produced by a picosecond prepulse radiation on the surface of different targets. High-order harmonics generated from the surface plasma of most targets showed a plateau pattern. It is assumed that the harmonic generation in these conditions occurs due to the interaction of the femtosecond pulses with the ions. The conversion efficiencies at the plateau region were varied between 1×10^-7 to 8×10^-6, depending on the target. The main contribution to the limitation of harmonic generation efficiency and cutoff energy was attributed to the self-defocusing of main pulse. A considerable restriction of the 27th harmonic generation was observed at different focusing conditions in the case of chromium plasma. Our observation of the resonance-induced enhancement of a single harmonic (λ=61.2 nm) at a plateau region with the efficiency of 8×10^-5 in the case of In plasma can offer some expectation that analogous processes can be realized in other plasma samples in the shorter wavelength range where the highest harmonics were achieved. PACS 42.65.Ky; 52.35.Mw; 52.38.-r     

Observations of light back-scattered from a laser-produced plasma

The back scattered light from a laser-produced plasma has been investigated. The laser consisted of a ruby oscillator and one amplifier producing a 10-nsec pulse of 10 joules. Observations of a Doppler-shifted component and of satellite lines are reported.     

Observations of extreme ultraviolet emission from plasma produced by capillary discharges

This study reports the results of a pilot experiment concerning observations of extreme ultraviolet emission from plasma produced by the capillary discharges. A few kA current was applied across the gas-filled alumina capillary (1 mm diameter and 8 mm long) to generate radiation in the EUV region (12–63 nm). Spectroscopic studies were carried out by means of a XEUV spectrometer which was upgraded for special lithography purposes. The results obtained from the EUV spectroscopic measurements provided information about the radiation processes from xenon and argon plasma and testifies that given capillary is an effective source of EUV emission. Additionally we showed a simulation which describes plasma dynamics parameters and dynamics of various ionization stages in capillary discharge. Our computer simulation confirmed the presence of ions, which spectra was registered in the experiment.     

Imaging using hard X-rays from a laser-produced plasma

Imaging of technical and biological objects using hard X-rays from a laser-produced plasma source is demonstrated. Magnification radiography and single-shot imaging of biological samples are feasible with the source, which utilised focused radiation from a short-pulse terawatt laser. Differential imaging with element specificity and a new projection geometry for X-ray radiography are proposed.     

Response of a SiC photodiode to extreme ultraviolet through visible radiation

The responsivity of a type 6H-SiC photodiode in the 1.5-400 nm wavelength range was measured using synchrotron radiation. The responsivity was 0.20 A/W at 270 nm and was less than 0.10 A/W in the extreme ultraviolet (EUV) region. The responsivity was calculated using a proven optical model that accounted for the reflection and absorption of the incident radiation and the variation of the charge collection efficiency (CCE) with depth into the device. The CCE was determined from the responsivity measured in the 200-400 nm wavelength range. By use of this CCE and the effective pair creation energy (7.2 eV) determined from x-ray absorption measurements, the EUV responsivity was accurately modeled with no free parameters. The measured visible-light sensitivity, although low compared with that of a silicon photodiode, was surprisingly high for this wide bandgap semiconductor.     

Nickel-like spectrum of platinum emitted from a laser-produced plasma

The X-ray spectrum of a laser-produced platinum plasma in the range 4.8–6.4 Å has been obtained. The lines belonging to the 3d-4p and 3d-4f transitions in the NiI isoelectronic sequence have been identified.     

极紫外光学器件辐照污染检测技术

总结并讨论了极紫外光刻技术中,有关极紫外光学器件受辐照污染的"在线"检测方法。简要介绍了极紫外光刻系统的原理、反射镜膜层结构以及表面污染产生的机理;指出光刻系统中"在线"检测的技术要求;分析了目前几种主要表面检测技术的特点;给出了每种方法在极紫外光学系统中的应用潜力;最后,指出光纤椭偏仪在极紫外光学系统的"在线"表面污染检测中具有良好的应用前景。     

XUV radiation transfer through high-Z metal foil in a laser-produced plasma

Aluminum and gold foils have been irradiated with 1.05-μm, 100-psec laser pulses at an intensity of 3 × 10¹⁴ W/cm². A spatially resolved spectrum of XUV radiation from a rear-side plasma has been observed in the wavelength range from 10 to 100 Å. The spectral intensity for the Al foil decays exponentially with foil thickness and goes to zero at 3 μm. The intensity for the Au foil also decays exponentially up to 1 μm but remains almost constant from 1 to 6 μm. This result for the Au foil indicates that radiation heat conduction plays an important role in energy transport through high-Z plasmas.     

激光等离子体极紫外光刻光源

研究并讨论了下一代光刻的核心技术之一—激光等离子体极紫外光刻光源。简要介绍了欧美和日本等国极紫外光刻技术的发展概况,分析了新兴的下一代13.5 nm极紫外光刻光源的现状,特别讨论了国内外激光等离子体极紫外光刻光源的现状,指出目前其存在的主要问题是如何提高光源的转化效率和减少光源的碎屑。文中同时概述了6.x nm（6.5～6.7 nm）极紫外光刻光源的最新研究工作。最后,介绍了作者所在研究小组近年来在极紫外光源和极紫外光刻掩模缺陷检测方面开展的研究工作。     

Magnetic-field confinement of a laser-produced plasma

Summary Ruby-laser pulses have been focused onto planar targets of beryllium in a magnetic field,B=17 kG. The field was oriented parallel to the direction of the incoming laser beam and perpendicular to the target surface. It was balancing the kinetic pressure of the plasma at a few millimeters from the target. Spectroscopic observations, both time integrated and time resolved, have shown the magnetic-confinement effect. In particular, a reduction of the transverse plasma-plume size and a density increase and slower expansion, in the presence of the magnetic field, have been observed. In the introduction, a short review of the most recent papers on the subject is reported.

---

Riassunto Gli impulsi luminosi di un laser a rubino sono stati focalizzati su bersagli piani di berillio in un campo magnetico di 17 kG. Il campo era orientato parallelamente alla direzione del fascio laser incidente e perpendicolarmente alla superficie del bersaglio ed era tale da bilanciare la pressione cinetica del plasma a qualche millimetro dal bersaglio. Osservazioni spettroscopiche, sia integrate che risolte nel tempo, hanno mostrato l’effetto di confinamento magnetico. In particolare, sono stati rilevati, in presenza del campo magnetico, una riduzione delle dimensioni trasversali del pennacchio di plasma ed un incremento della densità con rallentamento dell’espansione del plasma. Nell’introduzione è riportata una breve rassegna dei lavori piú recenti sullo stesso argomento.

---

Резюме Импульсы рубинового лазера фолусируются на плоские мишени бериллия в мапитном поле,B=17 кГс. Магнитное поле ориентировано параллельно направлению падающего лазерного лучка и перпендикулярно поверхности мишени. Кинетическое давление плазмы уравновешивается в нескольких миллиметрах от мишени. Спектроскопические наблюдения указывют на зффект мапнитнопо В частности, наблюдаются умеиьшение поперечнопо поперечнопо размера плазмы, увеличение плотности и расшение расширения в присутствии мапнитнопо поля. Во введении предлапается краткий обзор недавних работ на эту тему.